1. Field of the Invention
The present invention relates to a high performance zoom lens assembly for use with, for example, a video camera.
2. Description of the Prior Art
In recent years, demands have been made to provide a video camera not only with an easy-to-operate feature and a quick-to-operate feature, but also with a capability of presenting high quality images and an availability of multi-functions. To satisfy those demands, a high-resolution compact imaging device of 1/3 inch in size has been widely employed, and a compact imaging device of 1/4 inch in size is currently in use. Along therewith, a high-performance compact and light-weight zoom lens having a large aperture ratio is strongly desired. However, not only is a reduction in cost desired, but a realization of a high-magnification zoom lens employing a reduced number of component parts without sacrificing the high performance is also desired.
However, when it comes to a high-magnification zoom lens, not only are an increased lens diameter and an increased lens length required, but an increased number of lens elements is also required for accomplishing a strict correction of aberrations, resulting in an increase in manufacturing cost and also in size and weight to such an extent that the resultant zoom lens is not suited for-use in a video camera for home use. Accordingly, the compact and light-weight zoom lens assembly currently largely employed in video cameras for home use and of a type comprising about 10 lens elements and having an F-number of about 1.4 to 2.0 has a zooming ratio of 6 to 8.
A prior art zoom lens assembly of the type referred to above as disclosed in, for example, U.S. Pat. No. 5,100,223, issued Mar. 31, 1992 (based on Japanese Patent Application No. 1-169295) will now be discussed with reference to FIG. 2.
FIG. 2 illustrates an optical system of the prior art zoom lens assembly for use with a video camera. The prior art zoom lens assembly comprises, in the order the left as viewed therein, a first lens group 21 forming an image forming unit, a second lens group 22 forming a zooming unit, a third lens group 23 forming a light condensing unit, a fourth lens group 24 forming a focusing unit and a glass plate 25 which is an equivalent to a quartz filter and the faceplate of the image device or the like. Reference numeral 26 represents an image forming plane.
The prior art zoom lens assembly shown in FIG. 2 operates in the following manner. The first lens group 21 fixed relative to the image forming plane 26 plays an image forming role, while the second lens group 22 is movable along the optical axis of the zoom lens assembly to vary the magnification which in turn varies the focal length of the zoom lens assembly as a whole. The third lens group 23, which is a fixed lens group, acts to collect rays of light dispersed by the second lens group 22, while the fourth lens group 24 movable along the optical axis plays a focusing role. A change in position of the image resulting from a movement of the second lens group 22 during a zooming operation is compensated for by a movement of the fourth lens group 24 to keep the image focused on the image plane 26 at all times.
The prior art zoom lens assembly of the type referred to above, however, has a problem in that it is difficult to correct aberrations over a zooming region and over a range of photo-taking distances, particularly, chromatic aberrations, when the zooming ratio of 10 is chosen and it is therefore difficult to produce a high quality image with the assembly.